Early filtration media were largely derived from naturally occurring fibers such as wool, cellulose, asbestos, or other fibers commonly found in nature. Today wool and cellulose still play an important role, although asbestos has long since been discarded for reasons of its toxicity.
With the advent of plastic polymers, new media based on synthetic fibers are possible, featuring enhanced properties for filtration. At the same time, new filtration problems have arisen, such as automotive interior cabin filtration, and room cleaners applied to improving air quality in homes. These applications are particularly difficult as a filtration problem, as they target very fine particulate into submicron size, while at the same time demanding very low pressure drop performance in consideration of limited fan capabilities, all within highly limited space constraints. The preferred filter construction in respect of small size expectation is a pleated filter, in order to achieve increased filtration area per unit volume.
The limited size of available area results in high air velocity within the filtration media, posing a difficult problem for the media designer. Typically, this has meant certain performance trade-offs. One of the most fundamental of filtration trade-offs is between particle capture efficiency on the one hand, and pressure drop on the other.
Typically, the less obtrusive the filtration media is to air flow, the higher the flow output from the system into which the filter is installed. Filtration efficiency must often be compromised to keep flow within acceptable limits, to obtain satisfactory air system performance.
In certain filtration media commonly applied to the aforementioned applications, low pressure drop is gained through use of relatively coarse fibers, typically 10 micron average diameter and greater. Both round and rectangular fibers have been applied, the latter commonly referred to as split fiber electret. These fibers are in turn electrostatically charged to enhance the level of particle capture efficiency for small particles, through the action of the electrostatic charge forces acting to attract these finer particles to the fibers. In practice these media have been found to lose their effectiveness as a function of time. In certain instances this occurs rapidly in the space of just days or weeks, particularly on exposure to elevated humidity and temperature, or on exposure to certain classes of aerosols, such as oily aerosols. The use of very thin media of low basis weight, comprising fine fibers in the range of 1 to 5 microns can significantly lower this tendency while still respecting the pressure drop demand, but at the expense of low loading capacity and thus much shortened filter life relative to the coarse fiber approach.
As increasing awareness of the hazards associated with certain airborne particulate increases the demand for filtration solutions, coupled with an increased concern over disposal frequency of used filters, designers are pushed to develop media possessing stable operating characteristics, combined with high loading capacity, while at the same time respecting the pressure drop limitations of these high volumetric flow applications. A need thus exists in the art for a filter media and method for manufacturing same, which can address the problems mentioned above.